In an exhaust gas recirculation valve, a valve body for adjusting the flow rate of a circulating exhaust gas is composed of a valve for opening and closing an exhaust gas passage and a rotation shaft for operating the valve. The rotation shaft is rotatably supported by a bearing section provided in a housing of the valve body. Incidentally, a valve seat is provided in the housing forming the exhaust gas passage, and the valve is rotated integrally with the rotation shaft and abut against the valve seat, thus choking the exhaust gas passage.
In a conventional exhaust gas recirculation valve, there is the following problem: when foreign matter such as deposit contained in an exhaust gas is deposited in the vicinity of the sealing faces between a valve and a valve seat, the valve and the valve seat stick to each other because of the bite of the foreign matter, thus hampering the opening and closing of the valve body.
In order to solve such a problem, Patent Document 1 discloses a butterfly valve used for an exhaust gas recirculation valve. In the butterfly valve, the outer peripheral surface of a circular valve is formed with a sloping face, a valve seat is composed of two components and has an annular difference in level provided therearound, and the outer peripheral surface of the valve is line-sealed with the annular difference in level of the valve seat over a wide range to choke an exhaust gas passage.
However, although the line seal is secured over the wide range in the above arrangement, in a closed state of the valve, the high pressure of the gas flowing in the exhaust gas passage acts on the rotation shaft of the valve to thereby apply a heavy load on a seal section in the vicinity of the shaft; when the seal is not formed, leakage from the corresponding section is increased. Further, the valve seat is composed of two components; thus, there is a drawback such that the arrangement thereof is complicated.
In order to cover such shortcomings, Patent Document 2 discloses a butterfly valve having a multiple-offset structure where the rotation axis of a valve is offset in the direction of thickness of the valve and the valve is completely circular in cross section.